Re: Measuring cpu time using getrusage

[Tim Schmielau <tim@physik3.uni-rostock.de>]

On Wed, 14 Feb 2007, Johann Schuster wrote:

> currently i'm trying to measure the cpu times for
> numerical algorithms written in C using two calls of
> getrusage() and then calculating the time difference.
> 
> According to the man page of getrusage i looked at the
> user time of my process. Surprisingly the measured user
> times vary strongly depending on the system load of the
> machine.
> A heavy system load nearly doubles the measured time
> for my routine.

Does your system have hyperthreading? (look at whether the siblings field 
in /proc/cpuinfo is greater than one)
If yes, there is no way to accurately measure runtimes because the 
(single-threaded) performance of your CPU depends on the other tasks 
running. Just disable hyperthreading for these measurements.

Tim